The use of synthetic water-soluble polymers as wet end additives for the strengthening of paper and paperboard is widely practiced. The use of cellulose reactive water-soluble vinylamide copolymers as paper strengthening agents is also common. One particular class of vinylamide polymer strength aids includes vinylamide polymers which are modified with glyoxal or cellulose reactive agents in such a way as to be thermosetting.
U.S. Pat. No. 3,556,392 describes the synthesis of glyoxal-reacted water-soluble vinylamide polymers used as paper strength agents. The vinylamide polymers can contain ionic comonomers or other comonomers which impart specific functionalities to the polymers to improve affinity to cellulose. The backbone vinylamide polymer is reacted with enough glyoxal to form a thermosetting adduct. The reaction is catalyzed by raising the pH of the reaction solution to approximately 8, and when a slight increase in solution viscosity is noted the pH is lowered to approximately 7 to slow the progress of the reaction. When a predetermined viscosity target is reached, the reaction is quenched by lowering the pH to approximately 3.5 to 4. The degree of functionalization of the vinylamide polymer with glyoxal is monitored by measuring the increase in viscosity of the reaction solution using Gardner-Holdt bubble viscometers. U.S. Pat. No. 3,556,392 teaches that following the final acid quench, when the desired extent of reaction has been reached, approximately half of the original glyoxal remains unreacted in the finished product and does not function as a strength aid.
U.S. Pat. No. 3,556,392 teaches the prior art finished product will form an insoluble gel when aged for 8 days at 73° F. and at a concentration of 9 percent solids.
U.S. Pat. No. 4,217,425 discloses a strength aid made from an aqueous blend of acrylamide homopolymer, polyDADMAC (polydiallyldimethyl ammonium chloride) and glyoxal. The reaction mixture is catalyzed by invoking mildly alkaline conditions and the solution viscosity is monitored until a predetermined increase in viscosity has been reached; at which time the reaction is “killed” by lowering the pH to approximately 4. In example 1 from U.S. Pat. No. 4,217,425, acrylamide polymer, DADMAC polymer and glyoxal are mixed in solution under alkaline conditions. After 360 minutes the solution viscosity is measured as 17 cps, after 400 minutes the viscosity is 32 cps and after 415 minutes the viscosity is 55 cps. The increase in molecular weight is measured as an increase in solution viscosity.
A paper strengthening agent made by glyoxalation of a cationic acrylamide polymer with a molecular weight in the range of 500 to 6000 is taught by U.S. Pat. No. 4,605,702. This patent alleges improved loss of wet strength over time compared to previous disclosures. A viscometer is used to measure the increase in the solution viscosity as the glyoxalation reaction progresses.
A glyoxalated-polyvinylamide which is alleged to have enhanced storage stability resulting from multiple additions of glyoxal and the addition of an aldehyde scavenger is taught in U.S. Publication No. 20050187356. The backbone polymer is glyoxalated at a pH of 8 until the viscosity reaches 12 cps, at which time the reaction pH is lowered to 7.1 to 7.2. The reaction continues at a moderate rate until a viscosity of 54 cps is reached, at which time the reaction is quenched by addition of sulfuric acid to lower the pH to about 3.5.
PCT Published Application No. 2006/016906 describes a cationic vinylamide crosslinked polymer which is treated with a cellulose reactive agent such as glyoxal to impart strength to paper.
U.S. Pat. Nos. 4,954,538, 5,041,503 and 5,320,711 teach microparticles of cross linkable, glyoxalated-polyvinylamide prepared by reverse phase microemulsion polymerization and describe adding glyoxal to the micro emulsion polymer to form a glyoxalated polymer.
The methods and products described above have clear disadvantages. The aqueous glyoxalated-polyvinylamide adduct formation described in the known art is monitored by following the increase in solution viscosity as the reaction progresses. If the reaction is allowed to move forward unimpeded, a water-insoluble gel will ultimately form. Glyoxalated microemulsions (U.S. Pat. Nos. 4,954,538, 5,041,503 and 5,320,711) contain significant organic carrier oils which are costly and give high volatile organic compounds (VOC). There are numerous applications where high VOC amounts restrict use.
The various glyoxalated-polyvinylamide adducts commercially available at present are commonly known to have an approximate shelf-life range of about four to six weeks, depending on the pH, concentration of the adduct polymer solution and temperature at the time of storage.
When the desired extent of glyoxalation is reached, approximately half of the original glyoxal remains unreacted in the finished product and does not function as a strength aid.
The inventor has discovered that unexpectedly, improved adducts are formed by the aqueous reaction of glyoxal and vinylamide polymer when the concentration of the vinylamide polymer during the reaction is close to a Critical Concentration which determines an inflection point(s) defined below.
Furthermore, the adducts formed by the inventive method are not limited by the constraints of poor storage stability, do not run the risk of gelation, contain less unconsumed glyoxal than products of prior processes and contain essentially no oils. Additionally, the glyoxalated vinylamides of the present invention impart improved wet and dry strengthening efficiency to paper and paperboard, when compared to adducts disclosed in previously described art.